Up to now, a variety of the so-called dry chemistry methods have been proposed in order to carry out the analysis of urea nitrogen in body fluids simply and rapidly without personal errors. A typical dry chemistry method uses an integral multilayer analytical element comprising a reagent layer containing urease and an alkaline buffering agent, an indicator layer for the detection of gaseous ammonia, and a selective permeation layer which is interposed between the reagent and the indicator layers and which permits only gaseous ammonia to pass therethrough.
For example, JP 1977-003488 A (Corresponding to U.S. Pat. No. Re. 30,267) discloses an integral analytical element having the fundamental multilayer structure described above. This analytical element uses a thin hydrophobic polymer layer as a selective permeation layer for gaseous ammonia.
JP 1983-077661 A discloses an integral multilayer analytical element for use in the analysis of ammonia or an ammonia-producing substance in liquid samples comprising a laminate of a transparent support, an indicator layer for gaseous ammonia, a liquid blocking layer, a reaction layer containing an alkaline buffering agent and a optionally reagent capable of producing ammonia by the reaction with the ammonia-producing substance, and a porous spreading layer in this order. The integral multilayer analytical element is characterized in that the liquid blocking layer is made of a porous substance comprising pores which function as air vents substantially cutting off liquid samples and permitting gaseous ammonia to pass therethrough under a condition of usage. In the multilayer analytical element, a membrane filter is used as a selectively transmissive layer to approve adhesion to the indicator layer and to give high sensitivity.
Further, JP 1992-157363 A discloses usage of polyvinyl alkyl ether etc. substantially free from ammonia and ammonium ion as an under coating on a support or a binder for an indicator layer to obtain an integral multilayer analytical element for use in the analysis of ammonia or an ammonia-producing substance in liquid samples with higher color development optical density, low color development optical density of background and higher measurement accuracy. JP 1992-157364 A discloses usage of a porous spreading layer containing poly-N-vinyl pyrrolidone and a binder for a reagent layer for an ammonia-producing reaction which does not contain substantially any ammonia, and does not generate ammonia or vary in its binder performance at a pH value of about 9 or more to obtain an integral multilayer analytical element for use in the analysis of ammonia or an ammonia-producing substance in liquid samples with higher color development optical density, low color development optical density of background and higher measurement accuracy.
Yet, the whole thickness of these conventional analytical slides is large since they have a thick porous membrane as a liquid blocking layer. Difference in thickness of these slides and other slides to analyze other analytes requiring no liquid blocking layer is so large that it is unstable to fabricate them on a same production line. However there is a limit in using a thinner porous membrane because of deterioration in its performance as a liquid blocker. Further, though there is a slide having a very thin liquid blocking layer using cellulose acetate butylate, an organic solvent is required to fabricate it, resulting in problems on equipments and environments.